Medical devices

ABSTRACT

Alloy compositions suitable for fabricating medical devices, such as stents, are disclosed. In certain embodiments, the compositions have small amounts of nickel, e.g., the compositions can be substantially free of nickel.

TECHNICAL FIELD

The invention relates to medical devices, such as, for example, stentsand stent-grafts.

BACKGROUND

The body includes various passageways such as arteries, other bloodvessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced, or even replaced,with a medical endoprosthesis. An endoprosthesis is typically a tubularmember that is placed in a lumen in the body. Examples of endoprosthesisinclude stents and covered stents, sometimes called “stent-grafts”.

Endoprostheses can be delivered inside the body by a catheter thatsupports the endoprosthesis in a compacted or reduced-size form as theendoprosthesis is transported to a desired site. Upon reaching the site,the endoprosthesis is expanded, for example, so that it can contact thewalls of the lumen.

The expansion mechanism may include forcing the endoprosthesis to expandradially. For example, the expansion mechanism can include the cathetercarrying a balloon, which carries a balloon expandable endoprosthesis.The balloon can be inflated to deform and to fix the expandedendoprosthesis at a predetermined position in contact with the lumenwall. The balloon can then be deflated, and the catheter withdrawn.

In another technique, a self-expandable endoprosthesis is formed of anelastic material that can be reversibly compacted and expanded, e.g.,elastically or through a material phase transition. During introductioninto the body, the endoprosthesis is restrained in a compacted conditionon a catheter. Upon reaching the desired implantation site, therestraint is removed, for example, by retracting a restraining devicesuch as an outer sheath, enabling the endoprosthesis to self-expand byits own internal elastic restoring force.

To support a passageway open, endoprostheses are made of materials, suchas low-carbon, austenitic stainless steel or Nitinol (a nickel-titaniumalloy), having appropriate mechanical properties, such as tensilestrength and yield strength. An example of a suitable stainless steel isUNS S31673, which is similar to AISI 316L but having a higher chromiumand nickel content range. UNS S 31673 has a general composition shown inTable 1:

TABLE 1 Composition of UNS S31673 Element Weight Percent Carbon 0.030maximum Manganese 2.00 maximum Phosphorus 0.025 maximum Sulfur 0.010maximum Silicon 0.75 maximum Chromium 17.00 to 19.00 Nickel 13.00 to15.00 Molybdenum 2.25 to 3.00 Nitrogen 0.10 maximum Copper 0.50 maximumIron Balancewhere the chemical composition is maintained such that % Cr+(3.3)(X %Mo)≧26.0. Materials such as UNS S31673, however, can be relativelyradiolucent. That is, the materials may not be easily visible underX-ray fluoroscopy, a technique used to locate and to monitor theendoprostheses during and after delivery. To enhance their visibility(e.g., by increasing their radiopacity), the endoprostheses can includea relatively radiopaque material, such as gold or platinum.

SUMMARY

The invention relates to medical devices, such as, for example, stentsand stent-grafts. In one aspect, the invention features a medical deviceincluding an austenitic and non-magnetic stainless steel alloy thatincludes a small quantity amount of nickel. For example, in someembodiments, the alloy is substantially free of nickel, which, as usedherein, means that the alloy has less than about one weight percent ofnickel. Nickel can cause an adverse (e.g., allergic and/or cytotoxic)effect in some subjects. At the same time, the alloy can provide themedical device with good radiopacity, tensile strength, yield strength,elongation, and/or resistance to corrosion. In some cases, the alloy hasa radiopacity, physical properties, and mechanical properties comparableor better than those of UNS S31673.

In another aspect, the invention features a medical device having analloy including iron and chromium, being substantially free of nickel,and having a radiopacity greater than the radiopacity of UNS S31673.

In another aspect, the invention features a medical device having analloy including iron, chromium, and less than five weight percent ofnickel. The alloy is fully austenitic and has a radiopacity greater thanthe radiopacity of UNS S31673. The alloy can have less than five weightpercent of nickel, e.g., less than four, three, two, or one weightpercent of nickel.

Embodiments may include one or more of the following features. The alloyis fully austenitic. The alloy, after annealing, has a tensile strengthgreater than about 490 MPa. The alloy, after annealing, has a yieldstrength of greater than about 190 MPa. The device alloy has a pittingresistance equivalent greater than about 26. The alloy further includesone or more elements selected from platinum, ruthenium, palladium,iridium, rhodium, gold, and/or osmium. The alloy includes between about0.5% and about 40% by weight of the first element. The device is in theform of a stent.

The alloy can have one or more of the following compositions. The alloyincludes between about 0.01% and about 1.0% by weight percent ofnitrogen, e.g., less than about 1.0% by weight of nitrogen. The alloyincludes between about 0.07% and about 55% by weight of cobalt, e.g.,between about 0.07% and about 32% by weight of cobalt. The alloyincludes between about 0.5% and about 20% by weight of manganese. Thealloy includes between about 0.03% and about 6% by weight of copper. Thealloy includes less than about 30% by weight of chromium, e.g., lessthan about 20% by weight of chromium. The alloy includes less than about3% by weight of molybdenum.

In another aspect, the invention features a medical device having analloy including iron, less than about 30% by weight of chromium, lessthan about 3% by weight of molybdenum, less than about 55% by weight ofcobalt, less than about 20% by weight of manganese, less than about 6%by weight of copper, less than about 0.03% by weight of nickel, lessthan about 1.0% by weight of nitrogen, and between about 0.5% and about40% by weight of a first element selected from platinum, ruthenium,palladium, iridium, rhodium, gold, and/or osmium, wherein the alloy issubstantially austenitic.

The alloy can have one or more of the following compositions. The alloyincludes between about 0.01% and 1.0% by weight of nitrogen. The alloyincludes between about 0.07% and about 32% by weight of cobalt. Thealloy includes between about 0.5% and about 20% by weight of manganese.The alloy includes between about 0.03% and about 6% by weight of copper.

In another aspect, the invention features a method of making a medicaldevice. The method includes selecting an alloy including iron, chromium,and less than 5% by weight of nickel, wherein the alloy is substantiallyaustenitic and has at least one of the following properties: aradiopacity greater than the radiopacity of UNS S31673, a tensilestrength, after annealing, greater than about 490 MPa, a yield strength,after annealing, greater than about 190 MPa, or a pitting resistanceequivalent greater than about 26; and incorporating the alloy in themedical device, such as a stent.

The alloy can have at least two of the properties, e.g., at least threeof the properties. The alloy can be substantially free of nickel. Thealloy can include between about 0.5% and about 40% by weight of a firstelement selected from platinum, ruthenium, palladium, iridium, rhodium,gold, or osmium.

In another aspect, the invention features a medical device including anickel-free alloy having the same structure (e.g., face centered cubic)and comparable mechanical properties as a stainless steel, such as UNSS31673, conforming to ASTM F 138, F 139, and ISO 5832-1 Composition D.The alloy can have enhanced radiopacity.

The alloys described herein can also be used in dental prostheses,jewelry, flatware, or other items that can come into bodily contact.

In yet another aspect, the invention features the alloy compositionsdescribed herein.

Other aspects, features, and advantages of the invention will beapparent from the description of the preferred embodiments thereof andfrom the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an embodiment of a stent.

FIG. 2A is a table showing the chemical compositions of four alloys;FIG. 2B is a table showing the Cr_(eq), Ni_(eq), and Cr_(eq)/Ni_(eq)ratios of the alloys of FIG. 2A; FIG. 2C is a table showing themicrostructure and New PHACOMP calculations for the alloys of FIG. 2A;and FIG. 2D is a table showing the calculated mechanical, corrosion, andradiopacity properties of the alloys of FIG. 2A.

FIG. 3A is a table showing the chemical compositions of six alloys; FIG.3B is a table showing the Cr_(eq), Ni_(eq), and Cr_(eq)/Ni_(eq) ratiosof the alloys of FIG. 3A; FIG. 3C is a table showing the microstructureand New PHACOMP calculations for the alloys of FIG. 3A; and FIG. 3D is atable showing the calculated mechanical, corrosion, and radiopacityproperties of the alloys of FIG. 3A.

FIG. 4A is a table showing the chemical compositions of six alloys; FIG.4B is a table showing the Cr_(eq), Ni_(eq), and Cr_(eq)/Ni_(eq) ratiosof the alloys of FIG. 4A; FIG. 4C is a table showing the microstructureand New PHACOMP calculations for the alloys of FIG. 4A; and FIG. 4D is atable showing the calculated mechanical, corrosion, and radiopacityproperties of the alloys of FIG. 4A

DETAILED DESCRIPTION

Referring to FIG. 1, a support 12 carries a stent 10, which is the formof a tubular member including struts 11 and openings 13. Depending onthe type of stent, e.g., balloon-expandable or self-expandable, support12 can be a balloon catheter or a catheter shaft.

Stent 10 is composed of an alloy based on an iron-chromium stainlesssteel. As shown in Table 2, the alloy generally includes, among others,a small quantity of nickel, chromium, iron, and an element X:

TABLE 2 Composition of alloy of stent 10 Element Weight Percent Nickel≦5.0 Chromium 15.00-20.00 Element X  0.50-40.00 Iron balance (e.g.,40-65)Element X can include one or more (e.g., two, three, four, five, six ormore) of the following elements, in any combination: platinum,ruthenium, palladium, iridium, rhodium, gold, and osmium. In addition,the alloy may include one or more of the following elements: carbon,nitrogen, manganese, copper, cobalt, and molybdenum.

As shown in Table 2, the alloy of stent 10 has a small quantity ofnickel. In embodiments, the alloy has less than about five percent byweight of nickel. For example, the alloy can have less than or equal to4%, 3%, 2%, or 1% by weight of nickel; and/or greater than or equal to1%, 2%, 3%, or 4% by weight of nickel. Preferably, the alloy issubstantially free of nickel, i.e., having less than or equal to 1% byweight of nickel (e.g., less than or equal to 0.05 or 0.03% by weight).It is believed that nickel can cause an allergic and/or cytotoxic effectin certain subjects. Therefore, by reducing the amount of nickel in thealloy of stent 10, the occurrence of the effect(s) can be reduced (e.g.,minimized or eliminated).

Nickel can be used to promote a stable austenitic microstructure in astainless steel alloy. It is believed that the austenite (face centeredcubic) structure provides the alloy that is non-magnetic with goodstrength and ductility, which, for example, is beneficial to stent 10because the stent can undergo considerable deformation during use. Thus,since the amount of nickel is relatively small, one or more otherelements capable of promoting and/or stabilizing an austeniticmicrostructure (“austenitizing elements”) can be added to provide astable austenitic structure in the alloy of stent 10.

Austenitizing elements include, for example, carbon, nitrogen,manganese, copper, cobalt, and certain element X (e.g., Pt, Ir, Rh, Ru,Os, and Pd). Carbon is capable of promoting and stabilizing austenite,but at high concentrations, carbon can react to form carbides, such asiron carbides, chromium carbides, and/or molybdenum carbides. The alloycan include up to 0.03 weight percent of carbon, e.g., less than orequal to about 0.02 or 0.01 weight percent. The alloy can includegreater than zero weight percent and less than about one weight percentof nitrogen, e.g., less than or equal to about 0.75, 0.50, or 0.25weight percent. Manganese can compose up to about 20 weight percent ofthe alloy, e.g., less than or equal to about 15, 10, or 5 weightpercent. In embodiments, such as when the alloy is used inballoon-expandable stents, the amounts of nitrogen and/or manganese arecontrolled so as to not significantly increase the strength, e.g., yieldstrength, which can hinder use of the stent. Copper, which can be anaustenite promoter and/or stabilizer, can be included up to six weightpercent, e.g., less than or equal to about five, four, three, two, orone weight percent. Cobalt, which can be an austenite promoter and/orstabilizer can be includes up to 55 weight percent, e.g., less than orequal about 55, 50, 45, 40, 35, 32, 30, 25, 20, 15, 10 or 5 weightpercent, and/or greater than or equal to about 5, 10, 15, 20, 25, 30,32, 35, 40, 45, or 50 weight percent. In some cases, the alloy includesless than one weight percent of cobalt.

Molybdenum can be added to the alloy to enhance the resistance of thealloy to corrosion, e.g., pitting and crevice corrosion. In embodiments,the alloy includes between about 2.25 to about 3.00 weight percent ofmolybdenum, e.g., greater than or equal to 2.3, 2.4, 2.5, 2.6, 2.7, 2.8,or 2.9 weight percent, and/or less than or equal to 3.0, 2.9, 2.8, 2.7,2.6, 2.5, 2.4, or 2.3 weight percent.

Chromium can also be added to the alloy to make the alloy more corrosionresistant. In embodiments, at 12 wt % or higher, chromium can form athin oxide layer on the surface of a steel that enhances the resistanceof the steel to corrosive attack. The degree of corrosion resistance isa function of the chromium concentration and the concentrations of otherelements in the steel. The alloy can include between about 5 and about30 weight percent of chromium. The alloy can include greater than orequal to 5, 8, 11, 14, 17, 20, 25, or 30 weight percent, and/or lessthan or equal to 30, 25, 20, 17, 14, 11, or 8 weight percent ofchromium. Increasing the concentration of chromium can improve thepitting resistance, e.g., to be equal to or better than UNS S31673. Butin some cases, the higher the chromium concentration, the moreaustenitizing elements and/or stabilizers, such as Co, Mn, N, and/or Cu,may be required to maintain an austenitic structure.

Element X is selected from a group of elements capable of enhancing theradiopacity of the alloy. Element X can be a face-centered-cubicelement. In embodiments, element X has a density equal to or greaterthan about 2 g/cc, e.g., equal to or greater than 9.9 g/cc. The alloycan include between about 0.5 and about 40 weight percent of element X.The alloy can include greater than or equal to about 5, 10, 15, 20, 25,30, or 35 weight percent, and/or less than or equal to about 40, 35, 30,25, 20, 15, 10, or 5 weight percent of element X. In some cases, such aswhen element X is rhodium, iridium, palladium, ruthenium, osmium, orplatinum, element X can also promote and/or stabilize the austenitestructure.

The alloy can include residual amounts of impurities elements. Forexample, the alloy may include residual amounts of phosphorus (e.g:,0.025 wt % maximum), silicon (e.g. 0.75 wt % maximum), sulfur (e.g.,0.010 wt % maximum), niobium (e.g., about 0.013 wt %), vanadium (e.g.,about 0.07 wt %), titanium (e.g., 0.002 wt %), and/or aluminum (e.g.,about 0.009 wt %). Other residual elements and residual amounts arepossible, which can be a function of the source of the materials.

Iron makes up the balance of the alloy of stent 10, e.g., afteraccounting for the other elements in the alloy described above. Incertain embodiments, the alloy includes between about 40 and about 65weight percent of iron.

Particular compositions of the alloy are selected to provide the alloywith one or more selected physical and mechanical properties, such asradiopacity, strength, elongation, and resistance to corrosion, suitablefor intravascular use. In embodiments, the physical and mechanicalproperties are comparable to or better than those of other stainlesssteels, such as UNS S31673, used in medical devices. Without wishing tobe bound by theory, it is believed that these properties can be modeledto help predict, and therefore, target, compositions can provide theselected properties. For example, a particular composition can beanalyzed to determine theoretically whether it can form a selectedphase, such as austenite. Similarly, the composition can be modeled todetermine theoretically whether is can have suitable mechanical andphysical properties for medical applications.

Microstructure: In some embodiments, the alloy has a microstructure thatis predominantly (greater than 50%) austenitic, i.e., the alloy isformed predominantly of the austenite phase. For example, the alloy canbe equal to or greater than 80%, 85%, 90%, or 95% austenitic.Preferably, the alloy is fully austenitic. As discussed above, it isbelieved that the austenite structure can provide a non-magnetic alloywith good strength and ductility, e.g., suitable for stent applications.

The microstructure of an alloy can be predicted using constitutionaldiagrams, such as the Schaeffler diagram and the Welding ResearchCouncil (WRC-1988) diagram. (See ASM International, ASM SpecialityHandbook: Stainless Steels, Welding, pp. 340-342, Davis J. R. Library onCongress Cataloging-In-Publication Data, 1994; Siewert et al., FerriteNumber Prediction to 100FN in Stainless Steel Weld Metal, Weld. J., Vol.67 No. 12, 1988, pp. 289s-298s; and Hull, Delta Ferrite and MartensiteFormation in Stainless Steels, Welding Research Supplement, May 1973,pp. 193-203.) The Schaeffler diagram predicts the phases in the alloy,and the WRC diagram provides more detail in the range underconsideration. In particular, the phase to which the alloy solidifiescan be dependent on the chromium equivalent to nickel equivalent ratio(Cr_(eq)/Ni_(eq)), in which:Cr_(eq)=(% Cr)+(% Mo)+(1.5)(% Si)+(0.5)(% Nb)Ni_(eq)=(% Ni)+(30)(% C)+(0.5)(% Mn)For a Cr_(eq)/Ni_(eq) ratio approximately 1.48 or less, the compositioncan solidify as austenite; for a Cr_(eq)/Ni_(eq) ratio approximatelybetween 1.48 and 1.95, the composition can solidify as a duplexstructure of austenite and ferrite; and for a Cr_(eq)/Ni_(eq) ratioapproximately 1.95 or greater, the composition can solidify as ferrite.In embodiments, it is desirable for the composition to solidify in theaustenite (A) phase or the austenite-ferrite (AF) phase, which is mostlyaustenite. In the ferrite (F) phase or ferrite-austenite (FA) phase(which is mostly ferrite), the solubility of nitrogen (which canincrease austenite formation and stability) can decrease as thecomposition solidifies, resulting in increased porosity.

Austenite stability at ambient temperature can also reduce theoccurrence of martensite formation in the alloy during cold formingoperations. Uncontrolled transformation of austenite to martensite canmake the alloy magnetic, can lead to dimensional instability, and can bethe dominant cause of work hardening, e.g., reduced ductility. Themartensite deformation temperature, M_(d), can be the temperature atwhich 50% of martensite is formed by 30% deformation. M_(d) can becalculated as follows:M _(d)(°C.)=13−462(C+N)−9.2Si−8.1Mn−13.7Cr−9.5Ni−18.5Mo−18.5Cu−10(Ru+Rh+Pd+Ir+Pt+Au)For more information, see Angel T., Formation of Martensite inAustenitic Stainless Steels: Effects of Deformation, Temperature, andComposition, Journal of the Iron and Steel Institute, May 1954, pp.165-174. In embodiments, the austenite phase in the alloy is stable athigh and low temperatures, and the formation of the intermetallic phasesat grain boundaries is reduced (e.g. minimized). In certain embodiments,M_(d) is well below zero degrees Celsius.

TCP Phases and Nitrogen Concentration: In embodiments, the alloyincludes reduced (e.g., minimal or no) amounts of brittle topologicallyclose packed (TCP) phases. A phase computational methodology, called“New PHACOMP” (Morinaga et al., Solid Solubilities inTransition-Metal-Base FCC Alloys, Philosophical Magazine A, 1985, Vol.51, No. 2, pp. 223-246), can be used to help predict the tendency ofaustenite to precipitate TCP phases, e.g., sigma (σ) phases innickel-based alloys, and cobalt and iron based superalloys.

In New PHACOMP, the d orbital energy level (Md) of an element is used tocalculate the average Md (Md^(ave)) for the composition using theformula:Md ^(ave)(eV)=ΣX _(i·)(Md)_(i)where X_(i) is the atomic fraction of element i in the composition, and(Md)_(i) is the Md of element i. The summation is taken over all theelements of the alloy.

It is believed that when Md^(ave) becomes larger than a critical value(Md^(crit)), phase instability can occur and a second phase, i.e., a TCPphase, is formed in the austenite matrix. Md^(crit) is a function of thesecond phase. Here, for compositions containing ≦0.06% nitrogen:Md ^(crit)(eV)=0.834+(6.25×10⁻⁵)Twhere T is the temperature in Kelvin. For compositions containing >0.06%nitrogen:Md ^(crit)(eV)=0.834+(6.25×10⁻⁵)T+0.02N_(max)More information can be found in Uggowitzer et al., High NitrogenAustenitic Stainless Steels—Properties and New Developments, InnovationStainless Steel, Florence, Italy, 11-14 October 1993.

In embodiments, the alloy has an Md^(ave) value equal to or greater thanMd^(crit), e.g., greater than by 0.002 eV.

In addition, New PHACOMP can be used to predict precipitation ofchromium nitride in relatively high-nitrogen stainless steels (e.g., asdescribed in Uggowitzer et al.). As discussed above, nitrogen can beadded to austenitic steels to stabilize austenite, e.g., by reducing theoccurrence of ferrite formation at high temperatures and martensitictransformation at low temperatures. In certain cases, when the nitrogencontent exceeds its solubility limit in austenite, chromium nitride(e.g., Cr₂N) can precipitate and deplete the matrix of chromium, therebyreducing passivity.

The Md^(ave) value (above) can be used to calculate this solubilitylimit. The solubility of nitrogen in the austenite phase can vary, e.g.,as a function of the composition. In embodiments, the estimatedsolubility limit at annealing temperatures (e.g., about 1050° C.) issimilar to that obtained after quenching. A formula to predict themaximum amount of nitrogen that can dissolve in austenite beforechromium nitride precipitates, N_(max)(%), is:N_(max)(%)=0.003exp{41660[(Md ^(ave)−0.75)/(2765−T)]}where T is the temperature in Kelvin. More information can be found atUggowitzer et al. In certain embodiments, the alloy has a nitrogenconcentration equal to or less than N_(max)(%) to reduce the occurrenceof chromium nitride precipitation.

Radiopacity: The alloy is preferably radiopaque. The radiopacity of thealloy can be enhanced by including one or more element X (e.g., Pt, Ir,Os, Re, Rh, Pd, Ru, and Au). Element(s) X that are good austeniteformers can also reduce the amount of other austenite forming orstabilizing elements (see above).

The effect of an element on the radiopacity of an alloy is dependent onthe relative proportion of the element, and the mass attenuationcoefficient of the element. The mass attenuation coefficient (μ/ρ) foreach element at various energy levels can be obtained fromhttp://physics.nist.gov/. (See, e.g., Hubbell, J. H. and Seltzer, S. M.(1997). Tables of X-Ray Mass Attenuation Coefficients and MassEnergy-Absorption Coefficients (version 1.03), available athttp://physics.nist.gov/xaamdi [2002, Nov. 5]. National Institute ofStandards and Technology, Gaithersburg, Md., which was originallypublished as NISTIR 5632, National Institute of Standards andTechnology, Gaithersburg, Md. (1995). See, also, NISTIR 5632, Tables ofX-Ray Mass Attenuation Coefficients and Mass Energy-AbsorptionCoefficients 1 keV to 20 MeV for Elements Z=1 to 92 and 48 AdditionalSubstances of Dosimetric Interest, Published date: May 1995.)

The linear attenuation coefficient of an element, at a certain energylevel, can be derived by multiplying its mass attenuation coefficient byits density. The average mass attenuation coefficient (μ/ρ)_(ave) of analloy can be obtained by multiplying the elemental mass attenuationcoefficient (μ/ρ) by the weight fraction of each element in the alloy,and summing the contribution of each element:

$\left( \frac{\mu}{\rho} \right)_{ave} = {\sum\limits_{i = 1}^{n}{\left( {{wt}\mspace{14mu}\%} \right)_{i} \cdot \left( \frac{\mu}{\rho} \right)_{i}}}$

The average density for the alloy can be calculated as:

$\frac{1}{\rho} = {\sum\limits_{i}\left( \frac{c_{i}}{\rho_{i}} \right)}$where c_(i) is the mass percent of element i, and ρ_(i) is the densityof pure element i.

The average linear attenuation coefficient (μ)_(ave) for the alloy canthen be obtained by multiplying the average mass attenuation coefficientby the average density. The radiopacity at a certain energy level can bederived as:Radiopacity=e^(μ) ^(ave) ^(x)where μ_(ave) is the average linear attenuation coefficient, and x isthe thickness of the alloy. Methods of calculating the radiopacity of analloy at a certain energy level are also described in Craig et al.,Development of a Platinum-Enhanced Radiopaque Stainless Steel (PERSS®),Stainless Steel for Medical and Surgical Applications, ASTM STP 1438, G.L. Winters and M. J. Nutts, Eds., ASTM International, Pittsburgh, Pa.,2002.

The radiopacity of the alloy is dependent on the incident energy and thethickness of the alloy. In embodiments, for an alloy sample, 0.005″(0.127 mm) thick, at an incident energy level of 40 keV, the alloy ofstent 10 has a radiopacity of equal to or greater than about 1.539. Atan incident energy level of 60 keV, the alloy of stent 10 can have aradiopacity of equal to or greater than about 1.156. At an incidentenergy level of 80 keV, the alloy of stent 10 can have a radiopacity ofequal to or greater than about 1.118. At an incident energy level of 100keV, the alloy of stent 10 can have a radiopacity of equal to or greaterthan about 1.069

For purposes of comparison, a stainless steel, such as UNS S31673(0.005″ thick″), has a radiopacity of 1.475 at 40 keV, 1.138 at 60 keV,1.066 at 80 keV, and 1.040 at 100 keV. These values are median valuesand can vary, depending on the particular composition.

In some embodiments, the alloy has a radiopacity greater than or equalto about 105%, 110%, 115%, 120%, or 125% of the radiopacity of UNSS31673 at 80 keV for a thickness of 0.005 inch; and/or less than orequal to about 130%, 125%, 120%, 115%, 110%, or 105% of the radiopacityof UNS S31673 at 80 keV for a thickness of 0.005 inch.

Mechanical Properties: The mechanical properties of an alloy can beestimated as follows.Tensile strength (MPa)=470+600(N+0.02)+14Mo+1.5δ+8d ^(−0.5)+20Ru+7Rh+9Pt+7Ir+12Pd+5AuYield strength(MPa)=120+210(N+0.02)−0.5+2Cr+2Mn+14Mo+10Cu+δ(6.15−0.054δ)+(7+35(N+0.02))d ^(−0.5)where d is the grain size (in mm), and δ is the delta ferrite content(in volume percent). In embodiments, d can be set at 0.04 mm and δ canbe set at zero percent. For more information, see Nordberg, MechanicalProperties of Austenitic and Duplex Stainless Steels, InnovationStainless Steel, Florence, Italy, 11-14 Oct. 1993, Vol. 2, pp.2.217-2.229; and Uggowitzer et al., Strengthening of AusteniticStainless Steels by Nitrogen, HNS-88.

In some embodiments, the alloy (after annealing) has a tensile strengthof equal to or greater than 490 MPa, e.g., greater than about 500, 600,700, or 800 MPa. Alternatively or in addition, the alloy (afterannealing) can have a yield strength of equal to or greater/less than190 MPa, e.g., greater than about 200, 300, or 400 MPa. Alternatively orin addition, the alloy (after annealing) can have an elongation equal toor greater than about 40%.

Corrosion Resistance: The corrosion resistance properties can also beestimated. In embodiments, a pitting resistance equivalent (PRE) of analloy is greater than or equal to 26 (e.g., for ASTM F 138 and 139, andISO5832-1). The pitting resistance equivalent can be predicted by usingthe formula:PRE=Cr+3.3Mo+16Nwhich accounts for the effect of nitrogen, which can have a beneficialeffect on pitting resistance. More information can be found in Gunn,Duplex Stainless Steels, Woodhead Publishing Limited, England, 1997, pp.84.

In embodiments, the alloy has a pitting resistance equivalent equal toor greater than about 26.

By using the models and methodologies described above, differentcompositions of alloys can be studied to determine whether a compositioncan provide one or more selected properties. For example, the models andmethodologies can be entered into a software program. A user can input aselected composition, and the program can output the predictedproperties of the composition, e.g., in tabular or graphical form. Theuser can select those compositions having predetermined, predictedproperties. The selected composition can have one or more (e.g., two,three, four, five, or more) of the properties described above, in anycombination.

The following examples are illustrative and not intended to be limiting.

EXAMPLES

FIGS. 2A-2D, 3A-3D, and 4A-D show seventeen alloy compositions (AlloysA-Q) and their physical, microstructural, and mechanical properties,predicted using the models and methodologies described above.

All of the compositions have Cr_(eq)/Ni_(eq) ratios of 1.48 or less,which indicate that the compositions can solidify to a phase containingan austenite phase. All of the compositions also have low martensitedeformation temperatures, M_(d), e.g., less than zero degrees Celsius.

New PHACOMP analyses indicate all of the alloys, except Alloys I and K-Oshould not precipitate sigma phases (TCP phases) because Md^(ave) isless than Md^(crit). Alloys L and M are predicted to precipitate sigmaphases. For Alloys I, K, N, and O, the tendency of the alloy toprecipitate TCP phases is borderline because the difference betweenMd^(ave) and Md^(crit) is less than 0.002. New PHACOMP analyses alsoindicate no precipitation of chromium nitride in the alloys because thenitrogen concentrations are less than the maximum amounts of nitrogensolubility, N_(max).

The predicted mechanical, corrosion, and radiopacity properties areshown in FIGS. 2D, 3D, and 4D.

Selected alloy compositions were manufactured using high-purity rawmaterials. The materials were melted in a button arc furnace in awater-cooled copper hearth under an argon atmosphere of approximately0.3 of an atmospheric pressure. The materials were homogenized bymelting three times, with turning between each melt. The alloys werethen annealed in a vacuum furnace, at between 1050° C. and 1150° C. forabout two hours.

Stent 10 can be formed by folding and welding a sheet or a foil of thealloy to provide a tube, e.g., using inert gas or electron beam methods,with appropriate protection against oxidation. The tube can then bedrawn or extruded to the desired diameter, or used to fabricate a stentdirectly. Alternatively, a thin-walled tube of the alloy can be used.Portions of the tube can be removed to provide the strut 11/opening 13arrangement. The portions can be removed by laser cutting, as described,for example, in U.S. Pat. No. 5,780,807. Alternatively, the portions canbe removed by electrochemical machining, electrical discharge machining,abrasive cutting/grinding methods, or photoetching. Stent 10 can then befinished by electropolishing to a smooth finish, by conventionalmethods. Stent 10 also can be annealed. In other embodiments, stent 10is made from a flat pattern that is then formed into a tubular shape byrolling the pattern to bring opposing edges together. The edges can thenbe joined, e.g., by welding.

In general, stent 10 can be of any desired shape and size (e.g.,coronary stents, aortic stents, peripheral stents, gastrointestinalstents, urology stents, and neurology stents). Depending on theapplication, stent 10 can have a diameter of between, for example, 1 mmto 46 mm. In certain embodiments, a coronary stent can have an expandeddiameter of from about 2 mm to about 6 mm. In some embodiments, aperipheral stent can have an expanded diameter of from about 5 mm toabout 24 mm. In certain embodiments, a gastrointestinal and/or urologystent can have an expanded diameter of from about 6 mm to about 30 mm.In some embodiments, a neurology stent can have an expanded diameter offrom about 1 mm to about 12 mm. An abdominal aortic aneurysm (AAA) stentand a thoracic aortic aneurysm (TAA) stent can have a diameter fromabout 20 mm to about 46 mm. Stent 10 can be balloon-expandable,self-expandable, or a combination of both (e.g., as described in U.S.Pat. No. 5,366,504).

Stent 10 can be used, e.g., delivered and expanded, according toconventional methods. Suitable catheter systems are described in, forexample, Wang U.S. Pat. No. 5,195,969, and Hamlin U.S. Pat. No.5,270,086. Suitable stents and stent delivery are also exemplified bythe NIR on Ranger® system, available from Boston Scientific Scimed,Maple Grove, Minn.

OTHER EMBODIMENTS

In other embodiments, stent 10 can include and/or be attached to abiocompatible, non-porous or semi-porous polymer matrix made ofpolytetrafluoroethylene (PTFE), expanded PTFE, polyethylene, urethane,or polypropylene. Stent 10 can include a releasable therapeutic agent ora pharmaceutically active compound, such as described in U.S. Pat. No.5,674,242, commonly-assigned U.S. Ser. No. 09/895,415, filed Jul. 2,2001, and U.S. Ser. No. 10/112,391, filed Mar. 28, 2002. The therapeuticagents or pharmaceutically active compounds can include, for example,anti-thrombogenic agents, antioxidants, anti-inflammatory agents,anesthetic agents, anti-coagulants, and antibiotics.

The alloy described above can also be used in other medical devices,e.g., endoprostheses. For example, the alloy can be used in filters suchas removable thrombus filters described in Kim et al., U.S. Pat. No.6,146,404; in intravascular filters such as those described in Daniel etal., U.S. Pat. No. 6,171,327; and vena cava filters such as thosedescribed in Soon et al., U.S. Pat. No. 6,342,062.

The alloy can also be used in guidewires such as a Meier Steerable GuideWire (for AAA stent procedure) and an ASAP Automated Biopsy Systemdescribed in U.S. Pat. Nos. 4,958,625, 5,368,045, and 5,090,419.

All publications, references, websites, applications, and patentsreferred to herein are incorporated by reference in their entirety.

Other embodiments are within the claims.

1. An endoprosthesis, comprising: a tubular member capable of supportinga body passageway, the tubular member being formed of a non-magneticalloy comprising at least 40% by weight of iron, from about 5% by weightto about 20% by weight of chromium, and greater than about 5% by weightof a first element having a density greater than 9.9 g/cc, the alloyhaving less than or equal to 1% by weight of nickel and having aradiopacity greater than the radiopacity of UNS S31673.
 2. Theendoprosthesis of claim 1, wherein the alloy is fully austenitic.
 3. Theendoprosthesis of claim 1, wherein the radiopacity is greater than about105% of the radiopacity of UNS S31673 at 80 keV for a thickness of 0.005inch.
 4. The endoprosthesis of claim 1, wherein the radiopacity isgreater than about 110% of the radiopacity of UNS S31673 at 80 keV for athickness of 0.005 inch.
 5. The endoprosthesis of claim 1, wherein theradiopacity is greater than about 115% of the radiopacity of UNS S31673at 80 keV for a thickness of 0.005 inch.
 6. The endoprosthesis of claim1, wherein the radiopacity is greater than about 120% of the radiopacityof UNS S31673 at 80 keV for a thickness of 0.005 inch.
 7. Theendoprosthesis of claim 1, wherein the radiopacity is greater than about125% of the radiopacity of UNS S31673 at 80 keV for a thickness of 0.005inch.
 8. The endoprosthesis of claim 1, wherein the alloy, afterannealing, has a tensile strength greater than about 490 MPa.
 9. Theendonrosthesis of claim 1, wherein the alloy, after annealing, has ayield strength of greater than about 190 MPa.
 10. The endoprosthesis ofclaim 1, wherein the alloy has a pitting resistance equivalent greaterthan about
 26. 11. The endoprosthesis of claim 1, wherein first elementis selected from the group consisting of platinum, ruthenium, palladium,iridium, rhodium, gold, and osmium.
 12. The endoprosthesis of claim 11,wherein the alloy comprises a plurality of first elements.
 13. Theendonrosthesis of claim 11, wherein the alloy comprises greater thanabout 5% by weight to about 40% by weight of the first element.
 14. Theendoprosthesis of claim 1, in the form of a stent.
 15. Anendoprosthesis, comprising: a tubular member capable of supporting abody passageway, the tubular member being formed of a non-magnetic alloycomprising at least 40% by weight of iron, from about 5% to about 20% byweight of chromium, greater than about 5% by weight of a first elementhaving a density greater than 9.9 g/cc, and less than four weightpercent of nickel, the alloy being fully austenitic and having aradiopacity greater than the radiopacity of UNS S31673.
 16. Theendoprosthesis of claim 15, wherein the alloy comprises less than aboutthree weight percent of nickel.
 17. The endoprosthesis of claim 15,wherein the alloy comprises less than about one weight percent ofnickel.
 18. The endonrosthesis of claim 15, wherein the radiopacity isgreater than about 105% of the radiopacity of UNS S31673 at 80 keV for athickness of 0.005 inch.
 19. The endoprosthesis of claim 15, wherein theradiopacity is greater than about 110% of the radiopacity of UNS S31673at 80 keV for a thickness of 0.005 inch.
 20. The endoprosthesis of claim15, wherein the radiopacity is greater than about 115% of theradiopacity of UNS S31673 at 80 keV for a thickness of 0.005 inch. 21.The endoprosthesis of claim 15, wherein the radiopacity is greater thanabout 120% of the radiopacity of UNS S31673 at 80 kcV for a thickness of0.005 inch.
 22. The endoprosthesis of claim 15, wherein the radiopacityis greater than about 125% of the radiopacity of UNS S31673 at 80 keVfor a thickness of 0.005 inch.
 23. The endonrosthesis of claim 15,wherein the alloy further comprises between about 0.01% and about 1.0%by weight percent of nitrogen.
 24. The endoprosthesis of claim 15,wherein the alloy further comprises between about 0.07% and about 55% byweight of cobalt.
 25. The endoprosthesis of claim 15, wherein the alloyfurther comprises between about 0.07% and about 32% by weight of cobalt.26. The endoprosthesis of claim 15, wherein the alloy further comprisesbetween about 0.5% and about 20% by weight of manganese.
 27. Theendoprosthesis of claim 15, wherein the alloy further comprises betweenabout 0.03% and about 6% by weight of copper.
 28. The endoprosthesis ofclaim 15, wherein the first element is selected from the groupconsisting of platinum, ruthenium, palladium, iridium, rhodium, gold,and osmium.
 29. The endoprosthesis of claim 28, wherein the alloycomprises a plurality of first elements.
 30. The endoprosthesis of claim28, wherein the alloy comprises greater than about 5% by weight to about40% by weight of the first element.
 31. The endonrosthesis of claim 15,wherein the alloy, after annealing, has a tensile strength greater thanabout 490 MPa.
 32. The endoprosthesis of claim 15, wherein the alloy,after annealing, has a yield strength of greater than about 190 MPa. 33.The endoprosthesis of claim 15, wherein the alloy has a pittingresistance equivalent greater than about
 26. 34. The endoprosthesis ofclaim 15, wherein the alloy comprises less than about 17% by weight ofchromium.
 35. The endoprosthesis of claim 15, wherein the alloycomprises less than about 14% by weight of chromium.
 36. Theendonrosthesis of claim 15, wherein the alloy further comprises lessthan about 3% by weight of molybdenum.
 37. The endoDrosthesis of claim15, wherein the alloy further comprises less than about 1.0% by weightof nitrogen.
 38. The endoprosthesis of claim 15, in the form of a stent.39. An endoprosthesis, comprising: a tubular member capable ofsupporting a body passageway, the tubular member being formed of anon-magnetic alloy comprising at least 40% by weight of iron, betweenabout 5% and about 30% by weight of chromium, less than about 3% byweight of molybdenum, less than about 55% by weight of cobalt, less thanabout 20% by weight of manganese, less than about 6% by weight ofcopper, less than about 0.03% by weight of nickel, less than about 1.0%by weight of nitrogen, and between about 0.5% and about 40% by weight ofa first element selected from a group consisting of platinum, ruthenium,palladium, iridium, rhodium, gold, and osmium, the alloy beingsubstantially austenitic.
 40. The endoprosthesis of claim 39, in theform of a stent.
 41. The endoprosthesis of claim 39, comprising aplurality of first elements.
 42. The endoprosthesis of claim 39, whereinthe alloy comprises between about 0.01% and 1.0% by weight of nitrogen.43. The endoprosthesis of claim 39, wherein the alloy comprises betweenabout 0.07% and about 32% by weight of cobalt.
 44. The endoprosthesis ofclaim 39, wherein the alloy comprises between about 0.5% and about 20%by weight of manganese.
 45. The endoprosthesis of claim 39, wherein thealloy comprises between about 0.03% and about 6% by weight of copper.46. A method of making an endourosthesis, the method comprising:selecting a non-magnetic alloy comprising at least 40% by weight ofiron, from about 5% to about 20% by weight of chromium, greater thanabout 5% by weight of a first element having a density greater than 9.9g/cc, and less than 4% by weight of nickel, the alloy beingsubstantially austenitic and having at least one of the followingproperties: (a) a radiopacity greater than the radiopacity of UNS S31673, (b) a tensile strength, after annealing, greater than about 490MPa, (c) a yield strength, after annealing, greater than about 190 MPa,or (d) a pitting resistance equivalent greater than about 26,incorporating the alloy in a tubular member of the endoprosthesis, thetubular member capable of supporting a body passageway.
 47. The methodof claim 46, wherein the alloy has at least two of the properties. 48.The method of claim 46, wherein the alloy has at least three of theproperties.
 49. The method of claim 46, wherein the alloy issubstantially free of nickel.
 50. The method of claim 46, wherein thealloy comprises between about 0.5% and about 40% by weight of a firstelement selected from a group consisting of platinum, ruthenium,palladium, iridium, rhodium, gold, and osmium.
 51. The method of claim46, wherein the endoprosthesis is a stent.
 52. The endoprosthesis ofclaim 1, wherein the alloy comprises between about 40% and about 60% byweight of iron.
 53. The endoprosthesis of claim 15, wherein the alloycomprises between about 40% and about 60% by weight of iron.
 54. Theendoprosthesis of claim 39, wherein the alloy comprises between about40% and about 60% by weight of iron.
 55. The method of claim 46, whereinthe alloy comprises between about 40% and about 60% by weight of iron.56. The endoprosthesis of claim 1, where in the alloy comprises fromabout 10% by weight to about 40% by weight of the first element.
 57. Theendoprosthesis of claim 15, wherein the alloy comprises from about 10%by weight to about 40% by weight of the first element.
 58. Theendoprosthesis of claim 39, wherein the alloy comprises from about 10%by weight to about 40% by weight of the first element.
 59. Theendoprosthesis of claim 46, wherein the alloy comprises from about 10%by weight to about 40% by weight of the first element.